1. Field of the Invention
The present invention relates to a method of detecting a focus of a projection optical system. More particularly, the present invention relates to a focus detecting method which may be suitably applied to the detection of a focus position of a projection optical system which is attached to a projection exposure apparatus used to produce a semiconductor integrated circuit, a liquid crystal panel, an imaging device (CCD), a thin-film magnetic head, or a magneto-optical disc.
2. Description of the Related Art
A projection exposure apparatus which is used to produce a semiconductor integrated circuit or a liquid crystal panel, for example, needs to strictly align a photosensitive substrate with the imagery position (focus position) of a projection optical system. To comply with the need, there have heretofore been proposed various methods in which an image of a mask pattern is received with a photoelectric sensor, and a focus position is obtained on the basis of an output signal from the photoelectric sensor.
The conventional methods may be roughly classified into two types. The first type of conventional focus position detecting method will be explained below with reference to FIGS. 9(a) and 9(b).
FIGS. 9(a) and 9(b) show one example of conventional focus position detecting methods. FIG. 9(a) schematically shows the arrangement of a projection exposure apparatus, and FIG. 9(b) is a graph showing the waveform of an output signal from a photoelectric detector (photoelectric sensor). As shown in FIG. 9(a), a reticle R1, which serves as a mask, has a pattern (reticle pattern) RPA having a light-transmitting portion. The reticle pattern RPA is illuminated with a bundle of illuminating light rays IL1 from an illumination system (not shown). An image of the pattern RPA is projected through a projection optical system PL1 onto a light-transmitting sensor pattern SPA which is provided on a wafer stage WS1, on which a photosensitive substrate is mounted. A ray bundle passing through the sensor pattern SPA is incident on a photoelectric sensor PES1 which is disposed directly below the sensor pattern SPA. In this example, the constituent elements have previously been positioned so that the image of the reticle pattern RPA which is projected by the projection optical system PL1 will be superimposed on the sensor pattern SPA. In addition, the constituent elements have been designed so that the projected image of the reticle pattern RPA and the sensor pattern SPA will substantially match each other in terms of shape and size. Here, a Z-axis is taken in a direction parallel to the optical axis of the projection optical system PL1, and an X-axis is taken in a direction parallel to the plane of FIG. 9(a) within a plane perpendicular to the Z-axis. Further, a Y-axis is taken in a direction perpendicular to the plane of FIG. 9(a).
In the above-described arrangement, if an output signal I from the photoelectric sensor PES1 is monitored with the wafer stage WS1 moved in the optical axis direction (Z-axis direction) of the projection optical system PL1, an output that is shown by the curve L1 in FIG. 9(b) is obtained. It should be noted that, in FIG. 9(b), the abscissa axis represents the position z in the direction Z, and the ordinate axis represents the output signal I of the photoelectric sensor PES1. When the projected image of the reticle pattern RPA and the sensor pattern SPA come into imagery relation to each other, that is, when the sensor pattern SPA comes at the focus position, almost all the ray bundle passing through the reticle pattern RPA passes through the sensor pattern SPA, and thus the output signal I of the photoelectric sensor PES1 reaches a maximum. As the distance from the focus position increases, the image of the reticle pattern RPA spreads on the sensor pattern SPA, resulting in a decrease in the quantity of light passing through the sensor pattern SPA. Accordingly, the output signal I from the photoelectric sensor PES1 decreases. Thus, the position BF1 in the optical direction of the sensor pattern SPA at which the output signal I reaches a maximum is detected as being a focus position.
The second type of conventional focus position detecting method is disclosed, for example, in U.S. Pat. No. 4,629,313, or Japanese Patent Unexamined Publication (KOKAI) No. 4-211110. The second method will be explained below with reference to FIGS. 10(a) to 10(e).
FIGS. 10(a) to 10(e) show another example of conventional focus position detecting methods. FIG. 10(a) schematically shows the arrangement of a projection exposure apparatus. FIGS. 10(b) to 10(d) respectively show the waveforms of outputs signals from a photoelectric sensor which are obtained when the position of the wafer stage WS1 in the optical axis direction is at three different points z.sub.1, z.sub.2 and z.sub.3. The projection exposure apparatus shown in FIG. 10(a) has an arrangement approximately similar to that of the projection exposure apparatus shown in FIG. 9(a). However, the arrangement in FIG. 10(a) differs from the arrangement in FIG. 9(a) in that the wafer stage WS1 is moved not in the direction Z but in a direction (direction X) which is perpendicular to the optical axis of the projection optical system PL1, and which is parallel to the plane of FIG. 10(a), and while doing so, the output signal I of the photoelectric sensor PES1 is monitored, and this operation is repeated with the position of the wafer stage WS1 in the optical axis direction varied to carry out measurement, thereby detecting a focus position. It should be noted that, in FIGS. 10(b) to 10(d), the abscissa axis represents the position x in the direction X, and the ordinate axis represents the output signal I of the photoelectric sensor PES1.
As shown in FIGS. 10(b) to 10(d), at the positions z.sub.1 and z.sub.3 in the direction Z, the deviation from the focus position is relatively large, and the image of the reticle pattern RPA is out of the focus position. Therefore, the waveform of the output signal I is relatively gentle, as shown by the curves L2 and L4. On the other hand, the position z.sub.2 in the direction Z is close to the focus position, and the image of the reticle pattern RPA is therefore sharp at the position z.sub.2. Accordingly, the waveform of the output signal I draws a curve of good contrast, as shown in by the curve L3 in FIG. 10(c). Differences between the maximum and minimum values of the output signals in FIGS. 10(b) to 10(d) are assumed to be C.sub.1 to C.sub.3, respectively.
FIG. 10(e) is a graph showing the relationship between the position z in the optical axis direction and the differences C.sub.1 to C.sub.3 defined as contrast C. The abscissa axis represents the position z in the direction Z, and the ordinate axis represents the contrast C. As shown by the curve L5 in FIG. 10(e), the contrast C draws a gentle curve which reaches a maximum at the position BF2 in the direction Z. In this case, the position BF2 at which the contrast is the highest is detected as being a focus position. It should be noted that there is another method in which comparison is made in terms of the angle at which each of the curves L2 to L4 rises in place of the contrast C.sub.1 to C.sub.3.
The above-described first method suffers, however, from some problems described below. If the image of the reticle pattern RPA is not closely superimposed on the sensor pattern SPA, the quantity of light entering the photoelectric sensor PES1 decreases, and thus the SN ratio of the signal deteriorates. Consequently, the reproducibility of detection of the focus position becomes degraded. In general, the signal becomes more sensitive as the reticle pattern or the sensor pattern becomes thinner, and moreover, it is desirable for the reticle pattern to have the same line width as that of the actual circuit pattern. For this reason, the reticle pattern is generally designed to have a small size which is close to the resolution of the projection optical system. More specifically, in an exposure apparatus for a semiconductor integrated circuit, for example, the line width of the reticle pattern is generally not larger than 1 .mu.m. Therefore, even if the pattern alignment has been strictly effected, the pattern position shifts with passage of time owing to the drift of the apparatus or other reason. Further, replacement of the reticle also causes the pattern position to shift because of errors in the reticle positioning accuracy or the pattern drawing accuracy. Therefore, pattern alignment is needed almost every time detection of a focus position is carried out. Accordingly, it takes a great deal of time to detect a focus position, causing the throughput (productivity) of the exposure process to be deteriorated.
In general, alignment is carried out by a method in which the light-receiving part of a photoelectric sensor is moved in a plane perpendicular to the projection optical system to search for a point at which the output of the photoelectric sensor reaches a maximum. Since the alignment is carried out before the detection of a focus position, the light-receiving part of the photoelectric sensor is not necessarily coincident with the image-forming plane. When the light-receiving part of the photoelectric sensor does not coincide with the image-forming plane, the image of the reticle pattern is not sharp, and the focus position cannot accurately be obtained.
In the second method, no strict alignment is required before the detection of a focus position, but it is necessary to effect scanning many times with the position of the wafer stage WS1 changed in the direction Z. Accordingly, the second method suffers from the problem that the throughput deteriorates.
In recent years, circuit patterns have become increasingly small and fine, and it has become necessary to align the photosensitive substrate with the focus position (i.e., to effect focusing) with a higher degree of accuracy. Projection exposure apparatuses have heretofore been adapted to automatically correct the focus position by arithmetically predicting a change of the focus position which may be caused, for example, by a change in the atmospheric pressure, absorption of exposure light by the projection optical system, or a change in the illumination method (e.g., a change of .sigma. value, which is a coherence factor, annular illumination, etc.). However, it is also necessary in order to effect even more strict focusing to detect a focus position by actual measurement. With the achievement of fine circuit patterns, the focus detecting operation by actual measurement must be carried out more frequently than in the past. Therefore, it is impossible to meet the demand with a focus position detecting method of low measuring efficiency.
An object of the present invention is to provide a method of detecting a focus of a projection optical system, which requires no strict alignment, and which enables a reduction in the time required for measurement.